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image showing several machined gears

How To Diagnose Gear Failures

Continuing our series on articles on how to diagnose different machinery failures, this page looks at the failure of gears. As before, analysing and diagnosing gear failures is all about recognising the visual signs associated with typical failure modes.

The first step is therefore to perform a careful inspection of the failed gear pair and their associated bearings. This page lists many of the typical failure modes for gears with their associated visual signs and potential causes.

Care must be taken to ensure the real root cause is found as more than one failure mechanism can sometimes lead to superficially similar damage.  It is important to note that nearly 50% of all gear failures are a result of their bearings and supporting arrangement.  The following additional article on solving gearbox problems discusses this aspect here.

Pictures of Some Typical Gear Failures

This page lists many typical gear failure modes with images showing the associated failure pattern. Some of the information on this page is taken from our Tribology Handbook – a PDF version is available from our site here.

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Pitting

gear tooth pitting failure

hardened gear pitch line pitting

Characteristics: Damage to the surface of gear teeth from cyclic contact stress transmitted through a lubrication film that is in or near the elastohydrodynamic regime. Because of the relative directions of surface sliding and movement of the contact load in gear teeth, which affects surface crack direction and propagation, pitting occurs preferentially in the dedendum of the teeth of both driving and driven gears.

Causes: The operating surfaces of gear teeth suffer pitting when subject to excessive contact stress. Serious progressive pitting, associated with continuous overloading will become steadily worse and probably lead eventually to tooth breakage (see later example).

Pitting of Unhardened Gears: With unhardened gears, pitting often occurs as a bedding in action and provides a corrective process for misalignment and minor profile errors. Such pitting often heals and the gear teeth can become smooth again after further running. Pitting of this nature will tend to start during the first few hundred hours of running and its incidence can be followed if it is monitored at regular intervals.

Pitting of Hardened Gears:
Any pitting of hardened gears is generally a cause for alarm since the problem is unlikely to improve with time. Particularly with case hardened gears if the case is thin or the core material weak the case may be crushed and flake off leaving large pits in the surface. Case hardened gears operating at high loads and low speeds also sometimes show pitting at their pitch lines. This appears to be associated with the reversing surface sliding shear stresses which occur at this position and subject the hardened case to local fatigue failure

Shock Overloading

Failure from high shock load

Characteristics: Gear tooth failure where fractured surfaces show a single brittle break with no sign of fatigue marking. Usually only one or two teeth are affected and, apart from the broken teeth, the others are in good condition with no sign of cracks.

Causes: Typically arises from some major torsional shock in the drive system, often associated with the failure of a machine driven by the gear box. Similar effects can also occur if a foreign hard body enters the gear mesh. This is generally obvious from the nature of the impression in the teeth and the gear blank, and the body which has caused the problem can generally be found in the failure debris in the bottom of the gearbox.

Tooth Bending Fatigue

typical gear fatigue failure

Failure from bending loads

Characteristics: The failure starts as a crack which is usually at the root of the tooth and proceeds across the base of the tooth until the complete tooth breaks away from the gear. When a failure arises from this cause there are often other adjacent teeth showing cracks at an earlier stage of development.

Causes: Failures are a result of excessive cyclic bending stresses applied to teeth in normal gear operation. As a result, this is one of the key considerations for the original gear design. Gears loaded with a uni-directional torque will typically show fatigue cracks growing from one side only. Bi-directionally loaded gears may show fatigue cracks growing inwards from both sides.

Fatigue From Pitting and Scuffing

fatigued gear tooth from scuffing and pitting

Characteristics: Tooth fatigue failure with additional evidence of pitting and/or scuffing on the failed gear teeth. Fatigue failure may not be from the root of the tooth as with typical bending fatigue.

Causes: Although fractures usually start at the root of teeth, surface pitting or scuffing can cause stress raisers on the surface can initiate fatigue cracks at other positions.

Breakage From Misalignment

Failure from misalignment

Characteristics: Gear tooth breakage at one end of a gear. Possible evidence of light scuff marks towards the end of the teeth.

Causes: Gear misalignment – Could be a gear mounting, manufacturing, bearing or shaft misalignment issue.

Scuffing

gear tooth scuffing

gear scuffing

hardened gear surface rippling

Characteristics: Scuffing is a severe type of adhesive wear which instantly damages tooth surfaces. It is characterised by scoring marks on the gear teeth where relative sliding motion occurs as shown in the accompanying images in this section. Scuffing damage on the tooth faces show a tendency to ridging at the pitch line as the sliding action of the teeth tends to drag the surface material towards this the pitch line (see first image).

Causes: Scuffing damage occurs on gear teeth if they are operated with an inadequate lubricant film between the teeth. High surface temperatures then arise from the frictional heating and local welding and surface dragging and scoring tend to occur. The surface damage is usually more severe on soft gear teeth. With hardened gear teeth the early stages of scuffing damage often show as surface ripples on the operating faces.

Plastic Flow of Gear Teeth

severe gear tooth plastic flow

gear tooth feather edge from plastic flow

Characteristics: The surfaces of unhardened the teeth become swaged, so that the surface material is squeezed out at the ends and towards the tips of the teeth.

Causes: Gear overload, particularly shock loads.

Abrasive Material in Lubricant

Surface embedding marks on gear teeth

severe erosion of gear teeth from long term exposure in contaminated oil

lapping wear of gear teeth

Characteristics: When hard abrasive material is present in the lubricant the teeth may show embedding damage or abrasive scoring sometimes leading to major surface wear.

Causes: Lubricant contamination.

Gear Tooth Corrosion

Corrosive pitting of gear teeth

Characteristics: The pits relating to this failure tend to be rounded and randomly distributed.

Causes: Water or condensation in a gearbox.